Methods and devices for making carbon nanotubes and compositions thereof

ABSTRACT

A method of making carbon nanotubes includes reacting hydrogen and carbon monoxide in a reaction chamber and in the presence of stainless steel. Typically, the carbon nanotubes are formed on the stainless steel. These carbon nanotubes can be removed from the stainless steel and can be used in a variety of applications.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 USC 119(e) of U.S.Provisional Patent Application No. 60/629,204, filed on Nov. 17, 2004,which provisional application is incorporated herein by reference in itsentirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The present inventions were supported, at least in part, underDAAD-19-01-1-0759; Grant No. 42501EL from the United States ArmyResearch Office. The Government of the United States may have certainrights in the inventions.

FIELD

The present inventions are directed to methods and devices for makingcarbon nanotubes, as well as the carbon nanotubes made therewith. Thepresent inventions are also directed to methods and devices for makingcarbon nanotubes in the presence of stainless steel, as well as thecarbon nanotubes made therewith.

BACKGROUND

Since their discovery in 1993, carbon nanotubes have attracted attentionbecause of their structural, mechanical, and electrical properties.Single-walled carbon nanotubes (SWNTs) have been synthesized using avariety of processes including laser vaporization, carbon arc discharge,and chemical vapor deposition (CVD). These methods typically formself-assembled bundles of individual tubes that have a range ofdiameters and chiralities (spiral angles). The diameters andchiralities, collectively as well as individually, affect the physical,chemical, optical, and electronic properties of the nanotubes. Thesenanotubes can be particularly useful in fields such as, for example,nanotechnology.

BRIEF SUMMARY

One embodiment of the inventions is a method of making carbon nanotubes.The method includes reacting hydrogen and carbon monoxide in a reactionchamber and in the presence of stainless steel. Carbon nanotubes areformed as a result of the reaction. Typically, the carbon nanotubes areformed on the stainless steel. These carbon nanotubes can be removedfrom the stainless steel and can be used in a variety of applicationssuch as electronic and mechanical applications, includingnano-electronic and nano-mechanical applications.

Another embodiment of the inventions is a composition including carbonnanotubes formed in the presence of stainless steel.

Yet another embodiment of the inventions is a device for forming carbonnanotubes. The device includes a chamber with one or more inlets forreceiving gas; and a stainless steel object disposed in the chamber uponwhich the carbon nanotubes are formed.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention aredescribed with reference to the following drawings. In the drawings,like reference numerals refer to like parts throughout the variousfigures unless otherwise specified.

For a better understanding of the present invention, reference will bemade to the following Detailed Description, which is to be read inassociation with the accompanying drawings, wherein:

FIG. 1 is a schematic diagram of a device for forming carbon nanotubes,according to the inventions;

FIG. 2 is a Raman spectrum of the G- and D-bands for carbon nanotubesformed in the presence of stainless steel, according to the inventions;

FIG. 3 is a Raman spectrum of the radical breathing mode region forcarbon nanotubes formed in the presence of stainless steel, according tothe inventions; and

FIGS. 4A and 4B are Raman spectra of the G- and D-bands and the radicalbreathing mode region for carbon nanotubes made under two different setsof conditions.

DETAILED DESCRIPTION

The present inventions are directed to the area of methods and devicesfor making carbon nanotubes, as well as the carbon nanotubes madetherewith. The present inventions are also directed to methods anddevices for making carbon nanotubes in the presence of stainless steel,as well as the carbon nanotubes made therewith.

Conventional methods for forming carbon nanotubes have generally beenunsatisfactory for a number of reasons. Chemical vapor deposition (CVD)methods typically have lower nanotube formation temperatures than othermethods, but CVD methods have generally required the use of nanoparticlecatalysts (often metallic nanoparticles). Although the particulatecatalyst can facilitate the formation of carbon nanotubes, theseparticles can also contaminate the nanotube product and can be difficultto remove. Moreover, the properties of the nanotubes can depend on thenanotube size distribution which is affected by the size distribution ofthe nanoparticle catalysts.

Instead of using added nanoparticle catalyst, stainless steel (forexample, bulk stainless steel) can be used to form carbon nanotubes byCVD. Although not wishing to be bound by any particular theory, it isbelieved that the stainless steel inherently has surface catalyticsites. The nanotubes are a reaction product of hydrogen and carbonmonoxide in the presence of the stainless steel, and, preferably, on thestainless steel. FIG. 1 illustrates one embodiment of a device forforming carbon nanotubes. The device 100 includes a chamber 102, one ormore inlets/outlets 104, 106, 108 for flow of gas into or out of thechamber, a stainless steel object 110, and a heating mechanism 112. Inwill be recognized that suitable devices may include a variety of otheritems including, for example, pressure gauges, temperature gauges andflow meters.

Generally, any chamber 102 suitable for CVD processes can be used, suchas quartz or stainless steel. The chamber 102 includes a heatingmechanism 112 to heat the interior of the chamber. One or moreinlets/outlets 104, 106, 108 are provided to allow for the flow of gas.In some embodiments, mixtures of gases can be provided through a singleinlet. In other embodiments, mixtures of gases can be provided by addingthe individual gases through separate inlets.

The stainless steel object 110 is placed in the chamber 102 and can beheld in place using clips, a platform, or the like or the stainlesssteel object can be suspended from the ceiling of the chamber. Thestainless steel object can have any shape such as, for example, a tube,rod, sphere, or cone. In other embodiments, the stainless steel objectcan be a piece of stainless steel held in another item. For example, abulk stainless steel object can be placed in the center of a tube, forexample, a quartz tube.

Any stainless steel can be used, principally due to its alloy nature. Inparticular, austenitic stainless steels are useful including, but notlimited to, 316 stainless steel. The inherent surface catalyst presentin stainless steel acts as catalytic sites for the growth of carbonnanotubes.

Generally, prior to adding the reactants for formation of the nanotubes,the chamber is purged using an inert gas or one of the reactant gases toremove air (and, in particular, oxygen) from the chamber. Examples ofsuitable inert gases include, but are not limited to, argon andnitrogen. The purging gas pressure is typically in the range of 0.5 to10 atmospheres (atm) (about 5×10⁴ to 1×10⁶ Pa). In at least someembodiments, this purging occurs for at least 30 minutes. Purging maynot be needed or may be used for a shorter period of time if the chamberhas not been exposed to air.

In at least some embodiments, during purging the chamber is heated tofurther degas the chamber. In one embodiment, the chamber is heated to atemperature in the range of 650 to 1200° C. In one embodiment, thechamber is heated at or near the reaction temperature.

After purging with an inert gas, the chamber is optionally purged withhydrogen. This second purging can occur for at least 5 minutes and,preferably, about 30 minutes or more.

The reactants, hydrogen and carbon monoxide, are then flowed into thechamber. The relative amounts of reactants can range from pure carbonmonoxide (100% CO) to 80% (by volume) H₂/20% CO. Typically the relativeamounts of reactants range from 40% H₂/60% CO to 20% H₂/80% CO. Thetotal pressure is typically at least 1 atm (about 10⁵ Pa). Generally,the total pressure is in the range of 1 to 10 atm (10⁵ Pa to 10⁶ Pa),but can be higher. If no hydrogen is provided at this point in theprocess, the carbon monoxide will react with the hydrogen used in theprevious hydrogen purging process.

The reaction temperature is typically at least about 650° C. Typically,the reaction temperature does not exceed 1200° C. Generally, thetemperature is in the range of 650° C. to 1200° C. and, preferably inthe range of 800° C. to 1000° C.

The reaction time can vary depending on factors such as the reactionmixture, reaction pressure, reaction temperature, size of the stainlesssteel object, size of the chamber, type of stainless steel, and therelative amounts of reactants. In at least some embodiments, thereaction time is at least 15 minutes and may extend 90 minutes or more.

After the reaction is completed, the gas mixture in the chamber can bechanged to pure hydrogen or an inert gas, such as argon, and this gascan flow at the reaction temperature for a period of time (e.g., 30minutes or more) to remove unreacted CO. The temperature of the chambercan then be slowly reduced to room temperature while the hydrogen orinert gas continues to flow for a period ranging from 30 minutes to 2hours or more.

The carbon nanotubes are typically formed as black, hair-like orpaper-like entities disposed on the stainless steel. The nanotubes canoften be brushed off the surface of the stainless steel to recover thenanotubes. The nanotubes can typically be purified by simple washingprocedures, such as refluxing under HNO₃/HCl (3:1) at ca. 90˜100° C.Carbon nanotubes formed using this method can have a relatively narrowdiameter distribution near 1 nanometer. The mean diameter of the carbonnanotubes can depend on the reaction temperature and ratio of hydrogento CO. In one embodiment, the mean diameter of the carbon nanotubes isin the range of 0.8 to 1.2 nm.

The nanotubes can be used in a variety of applications. Suchapplications include, but are not limited to, use in electronic andmechanical devices such as nano-electronic and nano-mechanical devices.

EXAMPLE

A laboratory constructed CVD chamber was used. A 316 stainless steeltube having a width of 25 mm and length of ca. 200 mm was positioned inthe CVD chamber. Highly purified (99.999%) argon gas at a pressure ofca. 1 atm was used to purge the chamber for more than 30 minutes toremove air. The chamber was raised to a temperature of 700° C. duringthe purging period.

After purging with argon, the chamber was filled with hydrogen to apressure of 1 atm and the chamber temperature was maintained at 700° C.for about 30 minutes. Next, a mixture of carbon monoxide and hydrogengas (4:1 by volume) was allowed to flow into the chamber until a totalpressure of about 5 atmospheres (about 5×10⁵ Pa) was reached. Thetemperature and pressure was maintained for 40 minutes during which thecarbon nanotubes grew on the stainless steel tube.

Next, the gas mixture entering the chamber was changed to pure hydrogenor argon (1 atm) and the temperature was maintained at 700° C. for about30 minutes. The reactor was then slowly cooled to room temperature (overca. a 1 h period) under the flowing inert gas or hydrogen. Black,paper-like carbon sheets of carbon nanotubes were removed from thesurface of the stainless steel tube.

The carbon nanotubes were characterized by microRaman spectroscopy usinga LabRam Raman spectrometer from Horiba Jobin Yvon, Edison, N.J. FIG. 2is a Raman spectrum of carbon nanotubes excited with 632 nm radiation.The Raman spectrum contains the D-band and G-band that arecharacteristic of carbon nanotubes. Specifically, the band at 1576 cm⁻¹is assigned to the G-band of ordered carbon, while the Raman band at1324 cm⁻¹ is attributable to disordered carbon (e.g., defects in thecarbon nanotubes).

FIG. 3 is a Raman spectrum in the radical breathing mode (RBM) regionand indicates that the nanotubes are single-walled nanotubes (SWNTs).Two strong peaks located at about 210.6 cm⁻¹ and 271.3 cm⁻¹ were foundconfirming that SWNTs were grown. The calculated mean diameters based onthese peaks are 1.12 and 0.86 nm, respectively, indicating that thesynthesized SWNTs have a narrow diameter distribution.

FIGS. 4A and 4B present additional Raman spectra for two differentreaction temperatures in both the D- and G-band and RBM regions,indicating the effect of temperature on carbon nanotube diameter andquality. FIG. 4A illustrates the growth of carbon nanotubes using astainless steel tube pretreated with hydrogen at 900° C. followed bygrowth of the nanotubes at 900° C. under pure CO at 7.5 atm (about7.5×10⁵ Pa) for 30 min. FIG. 4B illustrates the growth of carbonnanotubes using a stainless steel tube pretreated with hydrogen at 1000°C. followed by growth of the nanotubes at 1000° C. under pure CO at 7.5atm (about 7.5×10⁵ Pa) for 30 min. Peaks in both sets of spectrademonstrate the existence of carbon nanotubes with mean diameters of0.82, 0.90, 1.06, and 1.2 nm. The differences in the spectra indicatedifferent distributions of these nanotubes in the samples. In bothcases, the quality of the carbon nanotubes was significantly improvedover those nanotubes associated with FIG. 3.

The above specification, examples and data provide a description of themanufacture and use of the composition of the invention. Since manyembodiments of the invention can be made without departing from thespirit and scope of the invention, the invention also resides in theclaims hereinafter appended.

1. A method of making carbon nanotubes, the method comprising: reactinghydrogen and carbon monoxide in a reaction chamber and in the presenceof stainless steel; and forming carbon nanotubes as a result of thereaction.
 2. The method of claim 1, wherein reacting hydrogen and carbonmonoxide comprises reacting hydrogen and carbon monoxide at atemperature of 650 to 1200° C.
 3. The method of claim 2, whereinreacting hydrogen and carbon monoxide comprises reacting hydrogen andcarbon monoxide at a temperature of 800 to 1000° C.
 4. The method ofclaim 1, wherein reacting hydrogen and carbon monoxide comprisesproviding hydrogen and carbon monoxide to the reaction chamber at apressure in the range of 1 to 10 atm.
 5. The method of claim 1, whereinreacting hydrogen and carbon monoxide comprises providing 20% to 100%,by volume, carbon monoxide and 0 to 80%, by volume, hydrogen.
 6. Themethod of claim 1, wherein the stainless steel comprises 316 stainlesssteel.
 7. The method of claim 1, further comprising purging the reactionchamber with inert gas prior to reacting the hydrogen and carbonmonoxide in the reaction chamber.
 8. The method of claim 1, furthercomprising providing only hydrogen or an inert gas to the reactionchamber after reacting the hydrogen and carbon monoxide.
 9. The methodof claim 1, wherein reacting hydrogen and carbon monoxide comprisesreacting hydrogen and carbon monoxide for at least 5 minutes.
 10. Themethod of claim 1, wherein the stainless steel comprises a unitary body.11. The method of claim 1, wherein forming carbon nanotubes comprisesforming carbon nanotubes on the stainless steel.
 12. The method of claim11, further comprising removing the carbon nanotubes from the stainlesssteel.
 13. The method of claim 11, further comprising providing hydrogento the chamber prior to adding carbon monoxide.
 14. The method of claim12, further comprising using the carbon nanotubes in an application. 15.The method of claim 14, wherein the application is a nano-electronic ornano-mechanical application.
 16. A composition comprising: carbonnanotubes formed in the presence of stainless steel.
 17. The compositionof claim 16, wherein the carbon nanotubes have a mean diameter in therange of 0.8 to 1.2 nm.
 18. The composition of claim 16, wherein thecarbon nanotubes are devoid of particulate catalyst.
 19. The compositionof claim 16, wherein the carbon nanotubes are formed on the stainlesssteel.
 20. A device for forming carbon nanotubes, comprising: a chamberwith one or more inlets for receiving gas; and a stainless steel objectdisposed in the chamber upon which the carbon nanotubes are formed. 21.The device of claim 20, wherein the stainless steel object is a unitarybody.
 22. The device of claim 20, wherein the stainless steel objectcomprises 316 stainless steel.
 23. The device of claim 20, wherein thestainless steel object comprises austenitic stainless steel.
 24. Thedevice of claim 20, wherein the stainless steel object is suspended inthe chamber.